The present invention relates to a process for recovering hydrolyzed copolymers of vinyl acetate and unsaturated carboxylic or sulfonic acid from an aqueous solution dissolving them.
Various polyvinyl alcohols are widely employed as sizing agents for fibers, and the waste water from scouring inevitably dissolves these polyvinyl alcohol sizing agents. The recovery of polyvinyl alcohol sizing agents from such a waste water is of advantage to the reduction of cost for sizing agent and the cleaning of waste water. As a conventional process for removing polyvinyl alcohols from the waste water of desizing, there are known a salting out method, a flocculent sedimentation method using polyvalent metal salts, a bubble separation method, an oxidation method, an absorption method, an insolubilization method by formalization and a biochemical treatment method. However, these known methods have the disadvantages that the degree of removal is generally low and moreover polyvinyl alcohols are decomposed or, even if recovered, the recovered polyvinyl alcohols contain other additives and cannot be reused as sizing agents.
As a result of investigation to use polyvinyl alcohols, the present inventor already found that modified polyvinyl alcohols having a degree of hydrolysis in the vinyl acetate units of 50 to 80% by mole which are obtained by alkali-hydrolyzing copolymers of vinyl acetate and unsaturated carboxylic or sulfonic acid, particularly those containing the unsaturated carboxylic or sulfonic acid units of 0.05 to 10% by mole, exhibit the excellent sizing effect on a hydrophobic fiber such as polyester filament yarn and a blended yarn of a polyester with a hydrophilic fiber such as cotton, viscose rayon or linen. However, there has never been yet proposed an effective process for recovering such particular modified polyvinyl alcohols from the desizing waste water.